Bifocal lenses are comprised of two or more areas, or zones, with different optical powers, including typically a far-power zone for distance vision, and a near-power zone for near or close up vision. The two zones may be subdivided into additional power zones in which case the lens may be called a multifocal lens.
The retinal image and the visual percept that results from it are dependent upon the light that enters the eye through the entrance pupil. In order for a bifocal contact lens to function properly the entrance pupil must be covered at least partly or, more effectively, completely by the far-power zone of the lens when the eye observes a distant object and covered at least partly or, more effectively, completely by the near-power zone of the lens when the eye observes a near object. This function can be accomplished by the principle of alternating vision in which a shifting action (typically vertical) or translation of the contact lens is made to occur in order to place one or the other zone in front of the entrance pupil as the eye alternates between viewing distance and near objects.
An alternative principle, known as simultaneous vision can be utilized whereby the lens is designed and fitted in such a way as to position part or all of both the far and near-power zones in front of the entrance pupil at the same time so that each contributes to the retinal image simultaneously. This type of lens requires no translation, but has the drawback that two images are viewed simultaneously. The present invention is not concerned with nontranslating lenses, although translation might be combined with simultaneous vision.
In rigid prism bifocal contact lenses the lower edge of the lens tends to rest on the upper margin of the lower lid. When the wearer views a distant object the far-power zone ideally is positioned to cover the entrance pupil of the eye, and the near-power zone is positioned below the entrance pupil. The lens is held in position by gravity and the downward force of the upper lid. For near vision tasks, the eye rotates downward and the contact lens ideally shifts upward relative to the eye, moving the near-power zone to a position in front of at least part of the entrance pupil in order to provide an optical correction for near vision.
Attempts have been made to design soft contact lenses which operate in a similar manner to that described above with hard lenses. However, soft bifocal contact lenses tend to be much larger than rigid contact lenses, usually between 13 and 15 mm in diameter, and often extend beyond the limbus of the eye. When a soft prism bifocal contact lens is worn, the thicker portion of the lens moves downward and lies beneath the lower lid. As a result, the lens is not supported or braced by the upper margin of the lower lid. Hence, the prism component is successful in moving a soft prism bifocal contact lens to the desired low position and controlling meridional rotation but is not successful in inducing vertical lens shifts as the eye looks back and forth between distant and near objects.
Presently available soft bifocal contact lenses do not have sufficient vertical shifting action to fulfil the alternating vision principle and thus do not provide acceptable vision for both distance and near viewing. Most soft bifocal contact lenses that are available today are of the concentric bifocal type, and operate on the principle of simultaneous vision. It is recognized that theses lenses do not provide good vision for both distance and near viewing and are only worn successfully by those who are willing to accept less than optimal vision.
Devices are known to induce a vertical shift in a soft bifocal lens. U.S. Pat. No. 6,109,749 describes a soft bifocal contact lens that has an integrally formed bevel to aid translation of the lens. The bevel portion has upper and lower shoulders which converge to form an extended bevel. The bevel does not form part of the optical portion of the lens. U.S. Pat. No. 5,635,998 shows a multifocal contact lens that has an ellipsoidal shape and a single prism, which in combination produce an elongated zone of contact between the base portion of the prism and the lower eyelid. U.S. Pat. No. 5,912,719 shows a lens that is comprised of palpebral (lid) bosses projecting locally from the external surface in the peripheral circumferential direction and with a crest line of limited dimensions. The crest line has a peak in its middle area.
U.S. Patent Application 20030016331 discloses a soft bifocal contact lens incorporating two or more prisms into the same lens, which operate together but with different structure and function. One of the primary prisms provides a desired lens vertical positioning on the eye during distance viewing and control of meridional rotation in the plane of the corneal perimeter, the limbus. In addition, a secondary prism has a base that extends forward from the adjacent lens surface and provides for vertical lens shifting, or translation, so that the desired optical power zone of the contact lens is moved in front of the entrance pupil of the eye at the desired time. Typically the lens contains a segmented bifocal area on one surface in which the far-power zone of the lens is uppermost and the near-power zone lowermost in position.
Accordingly, devices are known for use with both soft and hard bifocal contact lenses for translating the lens automatically to provide for the desired optical power zone of the lens to be in front of the entrance pupil of the eye at the desired time. One problem, however with translation type bifocal contact lenses is that in the absence of external forces such as those caused by interference of eyelids, a lens placed on the eye will tend to move to a position of minimum potential energy. This process is commonly referred to as “centration”. Potential energy, in the sense used in this specification is determined by a combination of gravitational forces, internal elastic forces, surface tension, pressure/suction under the lens, and lens/tear film/eye interaction. The contribution from interaction with the eyelids is excluded from the concept of potential energy.
Although translation or displacement of the lens is a three-dimensional quantity (x, y, and rotation), complicated further by distortion of the lens, for the purposes of this specification it will be treated as a one-dimensional variable (generally representing vertical displacement) except where otherwise noted.
Displacing a lens from the minimum-energy position requires the application of an external force. Displacement increases the potential energy of the lens, and so when the displacing force is removed the lens tends to return to the position of minimal energy, that is, it re-centres itself. Ideally, the lens optics are reconciled with shape so that this minimum-energy position provides the desired optical correction. Of course, with translation type bifocal contact lenses, as discussed above, the lens has two positions where it must provide the desired optical correction, one corresponding to near vision, and the other to distance vision.
In prior art lenses, in one or other of the positions, the lens is not in its minimum potential energy position. Holding the lens away from its minimum-energy position requires the presence of an externally applied force., such as interaction with the eyelid, which must be maintained for as long as the lens is to stay in this off-centre position. The application of this force can potentially cause discomfort or mechanical injury to the delicate structures of the eye.
Furthermore, any variation in the position of the eyelid is likely to change the force acting on the lens, and hence alter its position on the eye, potentially interfering with optical performance.